Const enums and optimization in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a const enum named Colors.

Typescript

const enum [1] {
  Red,
  Green,
  Blue
}

Drag options to blanks, or click blank then click option'

AEnumColors

BColorList

CPalette

DColors

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different name than the one declared.

Forgetting to use 'const' before enum.

The enum name should be Colors as declared.

2fill in blank

medium

Complete the code to access the enum value for Green.

Typescript

const greenValue = Colors.[1];

Drag options to blanks, or click blank then click option'

ABlue

BGreen

CRed

DYellow

Attempts:

3 left

💡 Hint

Common Mistakes

Using a member name that does not exist in the enum.

Using incorrect casing for the member name.

Accessing the Green member of the Colors enum returns its value.

3fill in blank

hard

Fix the error in the code to correctly use a const enum value in a function.

Typescript

function getColorName(color: Colors): string {
  return color === Colors.[1] ? 'Green' : 'Unknown';
}

Drag options to blanks, or click blank then click option'

ABlue

BRed

CGreen

DYellow

Attempts:

3 left

💡 Hint

Common Mistakes

Using a member that does not match the returned string.

Using a non-existent enum member.

The function compares the input to Colors.Green to return the correct name.

4fill in blank

hard

Fill both blanks to create a const enum and use its value in a variable.

Typescript

const enum [1] {
  Small = 1,
  Medium = 2,
  Large = 3
}

const sizeValue = [2].Medium;

Drag options to blanks, or click blank then click option'

ASizes

BColors

DShapes

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for the enum declaration and usage.

Using unrelated enum names.

The enum is named Sizes and the variable accesses Sizes.Medium.

5fill in blank

hard

Fill all three blanks to declare a const enum, assign a value, and compare it in a condition.

Typescript

const enum [1] {
  Low = 0,
  Medium = 1,
  High = 2
}

const currentLevel = [2].High;

if (currentLevel === [3].High) {
  console.log('High level reached');
}

Drag options to blanks, or click blank then click option'

APriority

DLevel

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for enum declaration and usage.

Mismatching enum names in condition and assignment.

The enum is declared as Level, but the variable and condition use Priority, which is incorrect. The correct consistent name is Priority for all blanks except the declaration which should be Level. The correct answer uses Level for declaration and Priority for usage, which is inconsistent. The correct consistent name for all is Priority. Here, the blanks are filled to show the correct usage with Priority.

Practice

1. What is the main benefit of using const enum in TypeScript?

easy

A. It allows enums to be modified at runtime.

B. It enables enums to be serialized to JSON automatically.

C. It replaces enum references with actual values at compile time, reducing runtime code.

D. It creates a separate JavaScript object for the enum at runtime.

Solution

1. Step 1: Understand what const enum does

   const enum replaces enum references with their actual values during compilation, so no enum object exists at runtime.

2. Step 2: Compare runtime behavior

   Unlike regular enums, const enum does not generate extra JavaScript code for the enum object, reducing code size and improving speed.

3. Final Answer:

   It replaces enum references with actual values at compile time, reducing runtime code. -> Option C

4. Quick Check:

   Const enums optimize code by replacing references with values [OK]

Hint: Const enums inline values, no runtime object created [OK]

Common Mistakes:

• Thinking const enums create runtime objects
• Believing const enums allow runtime modification
• Confusing const enums with regular enums

2. Which of the following is the correct syntax to declare a const enum in TypeScript?

easy

A. enum Colors { Red, Green, Blue }

B. const enum Colors { Red, Green, Blue }

C. constant enum Colors { Red, Green, Blue }

D. enum const Colors { Red, Green, Blue }

Solution

1. Step 1: Recall the correct keyword order

   The correct syntax uses the keyword const before enum to declare a const enum.

2. Step 2: Check each option

   const enum Colors { Red, Green, Blue } matches the correct syntax: const enum Colors { Red, Green, Blue }. Options C and D use invalid keywords or order.

3. Final Answer:

   const enum Colors { Red, Green, Blue } -> Option B

4. Quick Check:

   Const enum syntax = const enum [OK]

Hint: Remember: const comes before enum in declaration [OK]

Common Mistakes:

• Writing 'constant' instead of 'const'
• Placing 'const' after 'enum'
• Omitting 'const' keyword

3. Given the following code, what will be the output when compiled and run?

const enum Direction {
  Up = 1,
  Down,
  Left,
  Right
}

const move = Direction.Left;
console.log(move);

medium

A. 3

B. undefined

C. Left

D. 2

Solution

1. Step 1: Understand enum value assignment

   The enum starts with Up = 1, so Down = 2, Left = 3, and Right = 4.

2. Step 2: Identify the value of Direction.Left

   It is 3, so move is assigned 3, and console.log(move) prints 3.

3. Final Answer:

   3 -> Option A

4. Quick Check:

   Enum Left value = 3 [OK]

Hint: Count enum values starting from assigned number [OK]

Common Mistakes:

• Assuming Left is 2 instead of 3
• Expecting string output 'Left'
• Confusing enum with object property

4. What is the error in the following TypeScript code using const enums?

const enum Status {
  Active = 1,
  Inactive = 0
}

console.log(Status[1]);

medium

A. Compile-time error: Cannot use reverse mapping with const enums

B. Runtime error: Status is undefined

C. No error, outputs 'Active'

D. Outputs 1

Solution

1. Step 1: Understand const enum limitations

   Const enums do not generate an object at runtime, so reverse mapping (accessing by index) is not supported.

2. Step 2: Identify the error cause

   Using Status[1] tries to access reverse mapping, causing a compile-time error.

3. Final Answer:

   Compile-time error: Cannot use reverse mapping with const enums -> Option A

4. Quick Check:

   Reverse mapping disallowed in const enums [OK]

Hint: Const enums have no reverse mapping, avoid index access [OK]

Common Mistakes:

• Expecting reverse mapping to work with const enums
• Thinking it outputs string 'Active'
• Ignoring compile-time errors

5. You want to optimize a TypeScript program by replacing a regular enum with a const enum. Which of the following changes will NOT work correctly after this replacement?

hard

A. Using enum values directly in expressions and comparisons.

B. Using enum values as keys in a switch statement.

C. Assigning enum values to variables for later use.

D. Accessing enum values by their numeric index (reverse mapping).

Solution

1. Step 1: Recall const enum behavior

   Const enums inline values and do not create runtime objects, so reverse mapping (index to name) is not available.

2. Step 2: Analyze each option

   Options A, B, and D work fine as they use the inlined values directly. Accessing enum values by their numeric index (reverse mapping). requires reverse mapping, which fails with const enums.

3. Final Answer:

   Accessing enum values by their numeric index (reverse mapping). -> Option D

4. Quick Check:

   Reverse mapping breaks with const enums [OK]

Hint: Const enums disallow reverse mapping, avoid numeric index access [OK]

Common Mistakes:

• Assuming all enum features work with const enums
• Using reverse mapping without error
• Confusing const enum with regular enum behavior